1. Field of the Invention
The present invention relates generally to a rotor of a vehicular alternating current generator adapted to be installed in a motor vehicle, truck or the like, and more particularly to a fitting structure between a rotary shaft and a pair of Lundell type pole cores of the rotor.
2. Description of the Related Art
Conventional rotors for vehicular alternating current generators include a pair of Lundell type pole cores each having an axial hole in which a rotary shaft is press-fitted, and a field coil wound on the pole cores. The rotors of this type will be sometimes called “Lundell type rotors”. In the Lundell type rotors, inner end faces of the pair of Lundell type pole cores are in close contact with each other for flux penetration. A typical example of such conventional rotors for vehicular alternating current generator is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2000-125528.
To achieve a firm securement between the pair of Lundell type pole cores and the rotary shaft, there is a frequently used technique in which a rotary shaft having a knurled groove on its outer peripheral surface is press-fitted in axial holes of the pair of Lundell type pole cores. With this press-fitting, inner peripheral surfaces of the axial holes undergo plastic deformation and are displaced into the knurled groove of the rotary shaft with the result that the Lundell type pole cores are firmly secured on the rotary shaft against rotation relative to the shaft.
However, a plastically deformed portion of the inner peripheral walls of the axial holes, as it moves into the knurled groove of the rotary shaft during press-fitting operation, is displaced not only in a circumferential direction and a radial inward direction, but also in an axial direction toward a downstream side of the press-fitting direction of the rotary shaft. With this axial displacement of the plastically deformed portion of the inner peripheral walls, the inner end face of one Lundell type pole core, which is located on a downstream side as viewed from the press-fitting direction of the rotary shaft, is forced to displace downstream in the press-fitting direction of the rotary shaft, tending to create an axial space or gap between the inner end faces of the Lundell type pole cores. The axial gap will increase the magnetic resistance of a field magnetic flux circuit.
To avoid this problem, an attempt may be made to keep the pair of Lundell type pole cores in a tightly clamped condition during the press-fitting operation. However, the attempted tight clamping would require a large-sized production machine and cause undesired plastic deformations on the Lundell type pole cores. This problem may be relieved by increasing the radial cross-sectional area of the knurled groove, which may however result in a reduction in anchorage strength between the rotary shaft and the Lundell type pole cores.